This invention relates to an organic electroluminescent device, which is adapted for use as a display device or a light-emitting device such as a spontaneous light flat display, especially an organic electroluminescent color display using an organic thin film as an electroluminescent layer.
In recent years, importance of interfaces between human beings and machines including multimedia-oriented commercial articles is exalted. For more comfortable and more efficient machine operations, it is necessary to retrieve information from an operated machine without failure simply, instantaneously and in an adequate amount. To this end, studies have been made on various types of display devices or displays.
As machines are now miniaturized, there is an increasing demand, day by day, for miniaturization and thinning of display devices. For instance, there is an inconceivable development with respect to the miniaturization of lap top-type information processors of the all-in-one type such as notebook-size personal computers, notebook-size word processors and the like. This, in turn, entails a remarkable technical innovation on liquid crystal displays for use as a display device for the processor.
Nowadays, liquid crystal displays are employed as an interface of a diversity of articles and have wide utility in the fields not only of lap top-type information processors, but also of articles for our daily use including small-sized television sets, watches, desk-top calculators and the like.
These liquid crystal displays have been studied as a key of display devices, which are used as the interface connecting a human being and a machine and cover small-sized to large capacitance display devices while making use of the feature that liquid crystals are low in drive voltage and power consumption. However, liquid crystal displays have the problems that they do not rely on spontaneous light and thus need a greater power consumption for back light drive than for liquid crystal drive, with the result that a service time is shortened when using a built-in battery, thus placing a limitation on their use. Moreover, the liquid crystal display has another problem that it has such a narrow angle of field as not to be suitable for use as a large-sized display device.
Furthermore, the liquid crystal display depends on the manner of display using the orientation of liquid crystal molecules, and this is considered to bring about a serious problem that its contrast changes depending on the angle even within an angle of field.
From the standpoint of drive systems, an active matrix system, which is one of drive systems, has a response speed sufficient to deal with a motion picture. However, since a TFT (thin film transistor) drive circuit is used, a difficulty is involved in making a large screen size owing to the pixel defects, thus being disadvantageous in view of the reduction in cost.
In the liquid crystal display, a simple matrix system, which is another type of drive system, is not only low in cost, but also relatively easy in making a large screen size. However, this system has the problem that its response speed is not enough to deal with a motion picture.
In contrast, a spontaneous light display device is now under study such as on a plasma display device, an inorganic electroluminescent device, an organic electroluminescent device and the like.
The plasma display device employs plasma emission in a low pressure gas for display and is suited for the purposes of a large size and large capacitance, but has the problem on thinning and costs. In addition, an AC bias of high potential is required for its drive, and thus, the display is not suitable as a portable device.
The inorganic electroluminescent device has been put on the market as a green light emission display. Like the plasma display device, an AC bias drive is essential, for which several hundreds of volts are necessary, thus not being of practical use.
In this connection, however, emission of three primaries including red (R), green (G) and blue (B) necessary for color display has been succeeded due to the technical development. Since inorganic materials are used for this purpose, it has been difficult to control emission wavelengths depending on the molecular design or the like. Thus, it is believed that full color display is difficult.
On the other hand, the electroluminescent phenomenon caused by organic compounds has been long studied ever since there was discovered a luminescent or emission phenomenon wherein carriers are injected into the single crystal of anthracene capable of emitting a strong fluorescence in the first part of 1960s. However, such fluorescence is low in brightness and monochronous in nature, and the single crystal is used, so that this emission has been made as a fundamental investigation of carrier injection into organic materials.
However, since Tang et al. of Eastman Kodak have made public an organic thin film electroluminescent device of a built-up structure having an amorphous luminescent layer capable of realizing low voltage drive and high brightness emission in 1987, extensive studies have been made, in various fields, on the emission, stability, rise in brightness, built-up structure, manner of fabrication and the like with respect to the three primaries of R, G and B.
Furthermore, diverse novel materials have been prepared with the aid of the molecular design inherent to an organic material. At present, it starts to conduct extensive studies on applications, to color displays, of organic electroluminescent devices having excellent characteristic features of DC low voltage drive, thinning, and spontaneous light emission and the like.
The organic electroluminescent device (which may be sometimes referred to as organic EL device hereinafter) has a film thickness of 1 xcexcm or below. When an electric current is charged to the device, the electric energy is converted to a light energy thereby causing luminescence to be emitted in the form of a plane. Thus, the device has an ideal feature for use as a display device of the spontaneous emission type.
FIG. 14 shows an example of a known organic EL device. An organic EL device 10 includes, on a transparent substrate 6 (e.g. a glass substrate), an ITO (indium tin oxide) transparent electrode 5, a hole transport layer 4, a luminescent layer 3, an electron transport layer 2, and a cathode 1 (e.g. an aluminium electrode) formed in this order, for example, by a vacuum deposition method.
A DC voltage 7 is selectively applied between the transparent electrode 5 serving as an anode and the cathode 1, so that holes serving as carriers charged from the transparent electrode 5 are moved via the hole transport layer 4, and electrons charged from the cathode 1 are moved via the electron transport layer 2, thereby causing the re-combination of the electrons-holes. From the site of the re-combination, light 8 with a given wavelength is emitted and can be observed from the side of the transparent substrate 6.
The luminescent layer 3 may be made of a light-emitting substance such as, for example, anthracene, naphthalene, phenanthrene, pyrene, chrysene, perylene, butadiene, coumarin, acridine, stilbene and the like. This may be contained in the electron transport layer 2.
FIG. 15 shows another example of an organic EL device. In an organic EL device 20, the luminescent layer 3 is omitted and, instead, such a light-emitting substance as mentioned above is contained in the electron transport layer 2, and thus, the organic EL device 20 is so arranged as to emit light 18 having a given wavelength from an interface between the electron transport layer 2 and the hole transport layer 4.
FIG. 16 shows an application of the organic EL device. More particularly, a built-up body of the respective organic layers (including the hole transport layer 4, and the luminescent layer 3 or the electron transport layer 2) is interposed between the cathode 1 and the anode 5. These electrodes are, respectively, provided in the form of stripes that are intersected in the form of a matrix. In this state, a signal voltage is applied to in time series by means of a luminance signal circuit 34 and a shift register-built in control circuit 35 so that light is emitted at a number of intersected points (pixels), respectively.
Such an arrangement as set out above is usable not only as a display, but also as an image reproducing apparatus. It will be noted that if the striped pattern is provided for the respective colors of R, G and B, there can be obtained a full color or a multi-color arrangement.
In a display device made of a plurality of pixels using the organic EL device, emitting organic thin film layers 2, 3 and 4 are usually sandwiched between the transparent electrode 5 and the metal electrode 1, and emission occurs at the side of the transparent electrode 5.
The organic EL device set out above still has problems to solve. For instance, upon application of the organic EL device to a color display, it is essentially required to stably emit primaries of R, G and B. At the present stage, however, there have never been reported, except green light emitting materials, red and blue materials that have stability, chromaticity, brightness and the like enough to apply to displays.
Especially, with respect to blue emission of good chromaticity, it has now been difficult to obtain stable emission owing to the generation of heat from the course of a thermal relaxation procedure involving light emission and the presence of singlet oxygen or the like.
Moreover, where a dye of high crystallinity is used, an oligomer is produced upon solidification. This leads to a longer emission wavelength, with the high possibility that there occurs a phenomenon where even if emission takes place, it ceases immediately.
Many studies have been made on the development of a novel blue light-emitting material. Along with the study and development of a novel substance, it is important to obtain stable emission by application of existing materials. Additionally, the use of a material that has been established to some extent from the standpoint of its behavior contributes greatly to the shortage of time in the study and development, thus indicating an index to the development of materials.
For instance, a coumarin-based laser dye with a high fluorescent yield can be applied to as a doping material for improving the color purity of green emission, and has now been reported as obtaining an emission as a blue light-emitting material. This is considered for the following reason: a coumarin-based, short wavelength fluorescent dye is usually high in crystallinity in the form of a simple substance and is not suited as a stable blue emission material in an amorphous form; and at present, an amorphous stable thin film can be obtained according to a co-deposition technique.
For instance, coumarin 450 has a maximum fluorescent wavelength in the vicinity of 446 nm and a chromaticity corresponding to blue among R, G and B. However, coumarin has no electron transportability or hole transportability, so that its characteristics as a luminescent material is apparently poorer in comparison with materials having electron or hole transportability.
Materials, typical of which are zinc metal complexes, enable one to obtain stable blue emission by forming a blue luminescent layer having electron transportability as a single hetero-type structure. However, when an applied voltage is increased in order to obtain a satisfactory brightness, emission predominantly occurs in a region of a good spectral luminous efficacy at an emission spectrum in the vicinity of 700 nm. Eventually, there arises the disadvantage that the chromaticity of blue emission is shifted and comes close to white emission.
Further, the life of an organic electroluminescent device is generally short, and studies for prolonging the life have been extensively made in various fields.
However, for practical application as a display, it is preferred that a half-life time from an initial brightness (of about 200 cd) is 10,000 hours or over. Such an endurance time cannot be obtained yet. This presents a serious problem to solve in order to put the organic electroluminescent devices to practical use.
An object of the invention is to provide an organic electroluminescent device, which is able to realize blue emission of good chromaticity in a high luminous efficiency and high brightness.
Another object of the invention is to provide an organic electroluminescent device, which is able to continue stable emission over a long time.
Under the circumstances in the art, we have made intensive studies on applications of existing materials whose natures are well known, thereby causing amorphous thin films capable of emitting luminescence of good chromaticity in high brightness to efficiently emit luminescence. This will lead to considerable shortage of time in study and development, to realization of full color arrangements including a color display and also to contribution to the prolonged life of the device.
More particularly, according to the invention, there is provided an organic electroluminescent device of the type which comprises an emission region made of an organic compound and is constituted of a built-up body made of organic substances and including the emission region, wherein a portion contacting a main emission region contains a bathophenanthroline derivative of the general formula
General Formula: 
wherein X and Y may be the same or different and independently represent a hydrogen atom except the case where a hydrogen atom is at the 2 or 9 position, a substituted or unsubstituted alkyl group except the case where a methyl group is at the 2 or 9 position, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted amino group, a halogen atom, a nitro group, a cyano group or a hydroxyl group provided that at least one of these groups is contained at an arbitrary position.
In the electroluminescent device of the invention, the bathophenanthroline derivative that is contained in a portion contacting the main emission region acts to block the transport of holes, so that emission is obtained through electron-hole re-combination in a hole transport organic material (i.e. a hole transport layer has such a structure serving also as a luminescent layer that is an electron-hole re-combination region), so that stable emission of high brightness, especially, blue emission, is enabled by low voltage drive. The bathophenanthroline derivative has emission properties. Accordingly, there can be obtained not only light emission from the hole transport layer, but also emission from the bathophenanthroline derivative. At least one of the above emissions can be obtained.
Although fabrication of an organic electroluminescent device, and particularly, an amorphous organic electroluminescent device of the low voltage drive, spontaneous emission and thin type, has been considered to be difficult in view of its structure due to the absence of an electron transport material with excellent non-luminous properties, the invention can provide an organic electroluminescent device wherein its hole transport layer serves also as a luminescent layer that is a re-combination region of electrons and holes and which has a device structure of a long life capable of continuing stable emission over a long time.
More particularly, when an organic electroluminescent device is so arranged as to comprise a hole transport layer as a luminescent layer, stable emission can be obtained in high brightness and high efficiency. Especially, this becomes more appreciable with respect to blue emission, enabling one to obtain a peak brightness of 10,000 cd/m2 or over by DC drive and a peak brightness, calculated as DC, of 55,000 cd/m2 by pulse drive with a duty ratio of {fraction (1/100)}.
Aside from the blue emission device, bluish green emission, red or yellow emission via doping, and the control in chromaticity by doping are possible. Thus, there can be fabricated an organic electroluminescent-in-blue device capable of blue emission with an excellent chromaticity in high brightness. Hence, the possibility and shortage in time of development of materials, and indices to designs of novel luminescent materials and electron transport material can be shown.